Implement MKNetworkOperation addHeader.

This adds one single header, but otherwise acts like addHeaders.
Implement setHeader.

Unlike addHeader, this will overwrite any existing header, rather than
appending to it.  This allows the use of customHeaders in MKNetworkEngine
as the default values, but overridden on a per-operation basis if necessary.
Merge pull request #330 from tipbit/addHeader

Implement MKNetworkOperation addHeader.