Patch contributed by Kasper Føns:

. Clear request properties from header fields such that they are not included as the response headers (See issue #382)
. Do not send request if calling getIfModifiedSince()
. 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
. Throw exception when calling getOutputStream and connection has already been sent
. Do not handle cookies (HTTPUrlConnection does not)
. Delete getValueAsText duplicate method on HeaderEntry and use getValue instead
. Remove unused getHeader(String) method from HeaderEntry
	Change on 2014/07/01 by tball <tball@google.com>
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=70330073