Allow setting content_for directly

[denisdefreyne]

Implements #1066.

In addition to what is currently possible,

content_for(:head) { _erbout << 'stuff' }

it is now possible to pass a string, rather than a block that changes _erbout, to #content_for:

content_for(:head, 'stuff')

Note that

content_for(:head) { 'stuff' }

will (still) not have any effect.

• Add tests
• Allow passing string as first arg
• Refactor

Documentation will be updated afterwards.

[denisdefreyne]

Refactoring idea: passing in @config is ugly. Perhaps having @_context always available could be useful (and the rendering helper would then always pass @_context through).

[sunshineco]

Erm, did you mean s/directory/directly/ in the title of this pull request?

[denisdefreyne]

@sunshineco Ergh, brain fart. Fixed!