attribute value normalization in Canonicalization Algorithm

[gtkirankumar31]

With Reference to https://www.w3.org/TR/xml/#AVNormalize
we need to append a space character (#x20) for a white space character (#x20, #xD, #xA, #x9) to the normalized value. But we are also replacing multiple spaces by single space.
Need a small change in encodeSpecialCharactersInAttribute fn.

Existing code:
.replace(/[\r\n\t ]+/g, ' ')
Code change required:

.replace(/[\r\n\t ]/g, ' ')

[cjbarth]

@gtkirankumar31 , I see that you created a PR for this issue, which wasn't merged. It would be helpful if the PR included a test suite. If you're willing do so that, please reply here and we'll reopen the PR and continue work on this.