Fixed consistency in using None for on_behalf

The `on_behalf` argument is optional in some methods, but not others. If passed as `None` to `listFolders`, it will fail. This PR fixes this problem.

Example request:
```xml
<?xml version="1.0" encoding="UTF-8"?>
<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/" xmlns:m="http://schemas.microsoft.com/exchange/services/2006/messages" xmlns:t="http://schemas.microsoft.com/exchange/services/2006/types">
   <s:Header>
      <t:RequestServerVersion Version="Exchange2010_SP1"/>
   </s:Header>
   <s:Body>
      <m:FindFolder Traversal="Shallow">
         <m:FolderShape>
            <t:BaseShape>Default</t:BaseShape>
         </m:FolderShape>
         <m:IndexedPageFolderView BasePoint="Beginning" Offset="0"/>
         <m:ParentFolderIds>
            <t:DistinguishedFolderId Id="inbox">
               <t:Mailbox>
                  <t:EmailAddress/>
               </t:Mailbox>
            </t:DistinguishedFolderId>
         </m:ParentFolderIds>
      </m:FindFolder>
   </s:Body>
</s:Envelope>
```

Example response:
```xml
<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">
   <s:Body>
      <s:Fault>
         <faultcode xmlns:a="http://schemas.microsoft.com/exchange/services/2006/types">a:ErrorSchemaValidation</faultcode>
         <faultstring xml:lang="en-US">The request failed schema validation: The 'http://schemas.microsoft.com/exchange/services/2006/types:EmailAddress' element is invalid - The value '' is invalid according to its datatype 'http://schemas.microsoft.com/exchange/services/2006/types:NonEmptyStringType' - The actual length is less than the MinLength value.</faultstring>
         <detail>
            <e:ResponseCode xmlns:e="http://schemas.microsoft.com/exchange/services/2006/errors">ErrorSchemaValidation</e:ResponseCode>
            <e:Message xmlns:e="http://schemas.microsoft.com/exchange/services/2006/errors">The request failed schema validation.</e:Message>
            <t:MessageXml xmlns:t="http://schemas.microsoft.com/exchange/services/2006/types">
               <t:LineNumber>1</t:LineNumber>
               <t:LinePosition>548</t:LinePosition>
               <t:Violation>The 'http://schemas.microsoft.com/exchange/services/2006/types:EmailAddress' element is invalid - The value '' is invalid according to its datatype 'http://schemas.microsoft.com/exchange/services/2006/types:NonEmptyStringType' - The actual length is less than the MinLength value.</t:Violation>
            </t:MessageXml>
         </detail>
      </s:Fault>
   </s:Body>
</s:Envelope>
```

Example Stacktrace:
```
  File "C:\Python27\lib\site-packages\EWSWrapper_py\EWSWrapper.py", line 1395, in listFolders
    raise e
WebFault: Server raised fault: 'The request failed schema validation: The 'http://schemas.microsoft.com/exchange/service
s/2006/types:EmailAddress' element is invalid - The value '' is invalid according to its datatype 'http://schemas.micros
oft.com/exchange/services/2006/types:NonEmptyStringType' - The actual length is less than the MinLength value.'
```