Custom percent-encoders: don't mutate base encoder

[aantron]

At the moment, `Custom mutates the safe character array of its underlying component kind:

ocaml-uri/lib/uri.ml

Lines 166 to 176 in b4a8375

	| `Custom ((component : component), safe, unsafe) ->
	let safe_chars = safe_chars_for_component component in
	for i = 0 to String.length safe - 1 do
	let c = Char.code safe.[i] in
	safe_chars.(c) <- true
	done;
	for i = 0 to String.length unsafe - 1 do
	let c = Char.code unsafe.[i] in
	safe_chars.(c) <- false
	done;
	safe_chars

This was added in #147. I'm not sure if this is intentional or not (cc @orbitz). I am currently abusing `Custom to build IRI-like percent encoders from existing component kinds (as a workaround for lack of #34). I don't want to modify the underlying encoders, though — I'd like both IRI and ASCII-only support for my own code, and, in addition, I fear about violating the assumptions of other code linked into the program.

[orbitz]

Whoops! No, this is unintentional!

[aantron]

Any word on a merge of this PR and release? This is needed for Dream (see aantron/dream#50). Of course, it can be worked around by not using ocaml-uri, or using it less, but I would prefer not to add code that exists in ocaml-uri to Dream as well.

[avsm]

This is obviously safe and very important to release. I'll merge and cut a release now.

[aantron]

Thanks @orbitz and @avsm!