Lots of bugfixes, fixes and cleanups for IosHttpUrlConnection!

- Bugfix: Clear request properties from header fields such that they are
not included as the response headers (See issue #382)
- Bugfix: Do not send request if calling getIfModifiedSince()
- Bugfix: Do not use the contentType field for setting the Content-Type
header when performing a request. The field is always null at that
point, as it is used for getting the Content-Type from the response
headers
- Fix: Set connected = true when sending request (See issue #383)
- Fix: Throw exception when calling getOutputStream and connection has
already been sent
- Fix: Do not handle cookies (HTTPUrlConnection does not)
- Cleanup: Delete getValueAsText duplicate method on HeaderEntry and use
getValue instead
- Cleanup: Remove unused getHeader(String) method from HeaderEntry