* Added get/setDateHeader() methods that works with custom date
headers
* Removed get/setDateInMilliseconds() methods because they can be
simply represented with a chained getTime() call
* Fixed the inconsistent behavior & contract of
HttpHeaders.getContentLength() which does not throw an exception when
the header is missing or ill-formatted unless a default value is
specified
* Added clearHeaders() and removeHeader() to HttpHeaders
* Tabs to spaces
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