Do not set content-type HTML by default

http package heuristics do a decent job of figuring out the content type
Wrapper that guesses content-type from path

resource ending in .json gets content-type application/json, .txt gets
text/plain; charset=utf-8, etc.

Not registered by default

fixes hraban/web#11
Do not compress data by default
Allow wrapping main server
Server.Close() returns error on failure
Revert "Do not set content-type HTML by default"

This reverts commit 296989edd986f43069fd2648f747c34fc3e123d2.

There is a good reason to set HTML by default: compression will work on
resources without explicit content-type header. At the time the
compression wrapper is run the http package has not yet determined the
content-type heuristically so it doesnt know that the page is safe to
compress. Not pretty but okay I guess.
Example of page compression