don't submit content-length header on GET requests

According to the HTTP spec the presence of the 'Content-Length' header signals that there is a message body (even if the length is 0).  Also, since according to the spec, GET requests do not allow message bodies and therefore should not include the 'Content-Length' header.

From RFC 2616, Section 4.3:

"The presence of a message-body in a request is signaled by the
   inclusion of a Content-Length or Transfer-Encoding header field in
   the request's message-headers. A message-body MUST NOT be included in
   a request if the specification of the request method (section 5.1.1)
   does not allow sending an entity-body in requests."