fix: formatter for html and special casing of content-type

[udayvir-singh]

FIX 1

Fixes error caused by html formatter running even when tidy is not available on the system. Also fixes docs related to disabling formatters.

This PR is a cleaner version of #165.

---

Reason why formatter should be set to false instead of nil:

nil means nothing, ie. absence of value; that's why you have how pass false instead which has a actual value 0 in binary which can be set to a variable and passed around.

Here's another analogy:

Let's say you asked me to put something in a box, and I put <nothing> in the box. Is the box now empty or does it contain nothing? There is no way for you to verify that.

FIX 2

Fixes #157 hardcoded lower casing of content-type.

[teto]

nil means nothing, ie. absence of value; that's why you have how pass false instead which has a actual value 0 in binary which can be set to a variable and passed around.

setting the value to nil is the standard way of removing a key from a table http://lua-users.org/wiki/StoringNilsInTables so I was surprised by the current behavior, which may be an oddity of tbl_deep_extend (maybe worth documenting upstream). I will test then merge

[udayvir-singh]

@teto

From lua-users wiki:

Lua tables make no distinction between a table value being nil and the corresponding key not existing in the table

In fact, you may think of {} as a table with all possible keys set to nil, and this still takes only a small finite amount of memory because all those keys having nil values are not explicitly stored.

So basically coming back to my original comment, the following two tables are indistinguishable:

local tbl1 = {}

local tbl2 = {
  a = nil,
  b = nil,
  c = nil,
  d = nil,
  ... -- all the way to infinity
}