Document HTTP request snippet’s special treatment of the Host header

For an HTTP 1.1 request to be valid it must contain a Host header. To
ensure that the HTTP request snippet is a valid HTTP 1.1 request a
Host header will always be included. If one is not present in the
request that’s being documented, the snippet will generate one
automatically. A side-effect of this is that a preprocessor that removes
the Host header will have no effect on the HTTP request snippet. This
commit updates the documentation to describe this behaviour.

Closes gh-134

docs/src/docs/asciidoc/customizing-requests-and-responses.adoc

@@ -77,6 +77,10 @@ different replacement can also be specified if you wish.
 `removeHeaders` on `Preprocessors` removes any occurrences of the named headers
 from the request or response.
 
+NOTE: For an HTTP 1.1 request to be valid it must contain a `Host` header. Therefore,
+irrespective of any preprocessing, the default HTTP request snippet will always contain a
+`Host` header.
+
 
 
 [[customizing-requests-and-responses-preprocessors-replace-patterns]]

docs/src/docs/asciidoc/documenting-your-api.adoc

@@ -388,7 +388,8 @@ call that is being documented
 
 | `http-request.adoc`
 | Contains the HTTP request that is equivalent to the `MockMvc` call that is being
-documented
+  documented. HTTP 1.1 requires a `Host` header. If you do not provide one via the
+  `MockMvc` API the snippet will add one automatically.
 
 | `http-response.adoc`
 | Contains the HTTP response that was returned