Changes

HTTP_Adapter: allow body to be "omit"

In HTTP not all requests have a body. At the moment we describe a
missing body with body := "". This is not 100% correct since the rest of
the code interprets this as a present body with zero length and will put
a content_length = 0 header line into the HTTP header, even in the GET
request. This will most likely be ignored by any HTTP server, but it is
still not 100% spec compliant.

Related: SYS#6824
Change-Id: Ifedc8c2a590835663d1ba0b08b1fe4d54bdd0fff