Upgrade to HttpClient 4.3.6

Upgrade HttpClient library to [4.3.6](https://www.apache.org/dist/httpcomponents/httpclient/RELEASE_NOTES-4.3.x.txt). This resolves an issue with the library [where null cookies could trigger NPEs](https://issues.apache.org/jira/browse/HTTPCLIENT-1544).
The only caveat about this upgrade is the following from the 4.3.6 [release notes](https://www.apache.org/dist/httpcomponents/httpclient/RELEASE_NOTES-4.3.x.txt):
```    
SSLv3 protocol is disabled by default
```