Allow whitespace between '=' and delimiter.

Currently, qt-mustache will not allow any whitespace between the =
signs and the start/end delimiter in set delimiter tags. It will
treat such whitespace as being part of the delimiters and yield the
error "Custom delimiters may not contain '=' or spaces.".

Apparently, even though the specification reads

    "The tag's content MUST be any two non-whitespace sequences
     (separated by whitespace) EXCEPT an equals sign ('=') followed
     by the current closing delimiter." [1],

apparently such whitespace should ignored. E.g. tags such as

    {{= [ ] =}}

should be allowed [2]. The spec is slightly ambiguos here I think,
but that is apparently the intention.

This patch changes `readSetDelimiter(..)` to ignore such whitespace.
As a result the "Pair with Padding" test case in `delimiters.json`
now passes, since it has such a set delimiter tag. The "Sections"
test case in the same file have one as well, and would also pass
now if we only handled standalone tags properly.

[1] https://github.com/mustache/spec/blob/v1.1.2/specs/delimiters.yml#L5-L7
[2] https://github.com/mustache/spec/issues/29