[Web] Always return -1 as body length in HTTPClientWeb.

Body length cannot be reliably retrieved from the web.
Reading the "content-length" value will return a meaningless value when
the response is compressed, as reading will return uncompressed chunks
in any case, resulting in a mismatch between the detected body size and
the actual size returned by repeatedly calling read_response_body_chunk.

Additionally, while "content-length" is considered a safe CORS header,
"content-encoding" is not, so using the "content-encoding" to decide if
"content-length" is meaningful is not an option either.

We simply must accept the fact that browsers are awful when it comes to
networking APIs.

doc/classes/HTTPClient.xml

@@ -41,6 +41,7 @@
 			<description>
 				Returns the response's body length.
 				[b]Note:[/b] Some Web servers may not send a body length. In this case, the value returned will be [code]-1[/code]. If using chunked transfer encoding, the body length will also be [code]-1[/code].
+				[b]Note:[/b] This function always returns [code]-1[/code] on the Web platform due to browsers limitations.
 			</description>
 		</method>
 		<method name="get_response_code" qualifiers="const">