HTTP header "Authorization" is missing when https_proxy is set in the yaml config.

I use following code to initiate the DFP client, it works just fine if I don't need proxy server.
```
def dfp_client(auth_file='./auth.yaml'):
    client = dfp.DfpClient.LoadFromStorage(auth_file)
    return client

client = dfp_client()
li_service = client.GetService('LineItemService', version='v201403')
...
...
```

However, the code breaks when the proxy server is set in auth.yaml
```
dfp:
  application_name: Awesome App
  client_id: MYCLIENT_ID
  client_secret: MYCLIENT_SECRET
  refresh_token: MYREFRESH_TOKEN
  network_code: 1234
  https_proxy: 'http://localhost:8888'
```

The error I've got is:
```suds.WebFault: Server raised fault: '[AuthenticationError.MISSING_AUTHENTICATION @ ]'```

And then I set the logging level to DEBUG, and I can see "Authorization" is missing in the suds.client headers.  "Authorization" header was there if I disable the proxy.

```
DEBUG:suds.client:headers = {'SOAPAction': '""', 'Content-Type': 'text/xml; charset=utf-8'}
DEBUG:suds.transport.http:sending:
URL: https://www.google.com/apis/ads/publisher/v201403/LineItemService
HEADERS: {'SOAPAction': '""', 'Content-Type': 'text/xml; charset=utf-8', 'Content-type': 'text/xml; charset=utf-8', 'Soapaction': '""'}
MESSAGE:
<?xml version="1.0" encoding="UTF-8"?><SOAP-ENV:Envelope xmlns:ns0="https://www.google.com/apis/ads/publisher/v201403" xmlns:ns1="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:tns="https://www.google.com/apis/ads/publisher/v201403" xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/"><SOAP-ENV:Header><tns:RequestHeader><tns:networkCode>5129</tns:networkCode><tns:applicationName>Awesome App (DfpApi-Python, googleads/1.0.6, Python/2.7)</tns:applicationName></tns:RequestHeader></SOAP-ENV:Header><ns1:Body><ns0:getLineItemsByStatement><ns0:filterStatement><ns0:query>WHERE id = :id LIMIT 500 OFFSET 0</ns0:query><ns0:values><ns0:key>id</ns0:key><ns0:value xsi:type="ns0:TextValue"><ns0:Value.Type>TextValue</ns0:Value.Type><ns0:value>155559071</ns0:value></ns0:value></ns0:values></ns0:filterStatement></ns0:getLineItemsByStatement></ns1:Body></SOAP-ENV:Envelope>
DEBUG:suds.client:HTTP failed - 500 - Internal Server Error:
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/"><soap:Body><soap:Fault><faultcode>soap:Server</faultcode><faultstring>[AuthenticationError.MISSING_AUTHENTICATION @ ]</faultstring><detail><ApiExceptionFault xmlns="https://www.google.com/apis/ads/publisher/v201403"><message>[AuthenticationError.MISSING_AUTHENTICATION @ ]</message><ApplicationException.Type>ApiException</ApplicationException.Type><errors xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="AuthenticationError"><fieldPath></fieldPath><trigger></trigger><errorString>AuthenticationError.MISSING_AUTHENTICATION</errorString><ApiError.Type>AuthenticationError</ApiError.Type><reason>MISSING_AUTHENTICATION</reason></errors></ApiExceptionFault></detail></soap:Fault></soap:Body></soap:Envelope>
DEBUG:suds.metrics:<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/"><soap:Body><soap:Fault><faultcode>soap:Server</faultcode><faultstring>[AuthenticationError.MISSING_AUTHENTICATION @ ]</faultstring><detail><ApiExceptionFault xmlns="https://www.google.com/apis/ads/publisher/v201403"><message>[AuthenticationError.MISSING_AUTHENTICATION @ ]</message><ApplicationException.Type>ApiException</ApplicationException.Type><errors xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="AuthenticationError"><fieldPath></fieldPath><trigger></trigger><errorString>AuthenticationError.MISSING_AUTHENTICATION</errorString><ApiError.Type>AuthenticationError</ApiError.Type><reason>MISSING_AUTHENTICATION</reason></errors></ApiExceptionFault></detail></soap:Fault></soap:Body></soap:Envelope>
sax duration: 1 (ms)
```