Speed up ERB expression BLOCK_EXPR regex

I noticed that during our application's boot we were spending more time
than desirable matching against this BLOCK_EXPR regex. This was slower
than necessary because even though this regex engine only cares about
the end of the string, Ruby's regex engine only works forward, and this
regex would match whitespace anywhere inside the string using two
ambiguous adjacent whitespace repetitions.

We don't expect untrusted code here (this is Ruby in our ERB which we
are going to execute), but with realistic strings this still causes
unnecessary backtracking.

This commit fixes the problem by removing the initial "\s*" (since this
is optional we don't need to capture it, we can just find the match
later if it exists) and replacing the following "\s+" with just "\s"
(similarly we don't care how many whitespace appear in a row, we can
just match later if there is more than one).

Testing against all the ERB expressions in our app this is 10x faster:
from 1.5 seconds to 0.1 seconds.

actionview/lib/action_view/template/handlers/erb/erubi.rb

@@ -42,7 +42,7 @@ def add_text(text)
             end
           end
 
-          BLOCK_EXPR = /\s*((\s+|\))do|\{)(\s*\|[^|]*\|)?\s*\Z/
+          BLOCK_EXPR = /((\s|\))do|\{)(\s*\|[^|]*\|)?\s*\Z/
 
           def add_expression(indicator, code)
             flush_newline_if_pending(src)