lib/rdoc/markup/to_label.rb (convert): Make labels cross-browser compatible.

Percent-encoded URL fragments prevent Firefox from jumping to their respective anchors. Wikipedia (MediaWiki) does something similar, with `.` instead of `-`.

There's precedent in `RDoc::{Alias,MethodAttr}#html_name` (54dbcf70) and `RDoc::Context::Section#aref` (431baa1b). This breaks backwards-compatibility, though.
Merge pull request #330 from blackwinter/make-labels-cross-browser-compatible

Make labels cross-browser compatible.
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Add test for #302

Numbers in word pairs is allowed but not tested in AttributeManager (but
is probably tested elsewhere).
Add missing RDoc::SingleClass#aref_prefix

This was missing and not implemented which caused errors for users that
documented methods via the singleton class.

Fixes #311
Refactor common setup in TestRDocSingleClass
TestRDocSingleClass does not use XrefTestCase
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Backfill tests for RDoc::Stats#undoc_params

Part of #321
Ignore splat when checking for documented params

Previously we did not account for the param sigils in the arguments which
caused the param to be marked as undocumented.

Now we strip the splat sigil so the param gets correctly marked as
documented.

Part of #321.
Ignore keyword param when checking documentation

See @9f79384

Part of #321
Backfill block param documentation check test

Part of #321
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Fixes #321
Mark standalone anchors in markdown as raw

This matches behavior of other markdown parsers but is limited in usefulness
as the anchor will be escaped if used inline.

Fixes #312
Remove needless backtracking

This caused ruby 1.8.7 to have an overflow in the regular expression
matcher as it continually tried to backtrack on uncaptured content.

Fixes #327
Really fix regular expression to avoid overcapture

Previously there was a bogus .* in the regular expression that matched
newlines which is bad and would cause RDoc to find the wrong comment
(which was fixed by another .* removed in the previous commit).

Now we remove these things properly.

Fixes #327 (for real).
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