http client: DELETE method should not use chunked encoding by default

this addresses #6185

According to RFC-2616 http://tools.ietf.org/html/rfc2616

Section 14.41 (Transfer-Encoding) states:

>   The Transfer-Encoding general-header field indicates what (if any) type of transformation has been applied to the message body in order to safely transfer it between the sender and the recipient. This differs from the content-coding in that the transfer-coding is a property of the message, not of the entity.

Section 4.3 (Message Body) states:

>   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. A server SHOULD read and forward a message-body on any request; if the request method does not include defined semantics for an entity-body, then the message-body SHOULD be ignored when handling the request.

Since a message body is permissible for the DELETE method, deciding whether the `transfer-encoding` header should be set by default comes down to general usage and acceptance. Since DELETE requests generally do not have a message body this header should not be added by default. This remains consistent with the docs:

> request.write(chunk, [encoding])
> Sends a chunk of the body. By calling this method many times, the user can stream a request body to a server--in that case it is suggested to use the ['Transfer-Encoding', 'chunked'] header line when creating the request.