Remove urldecode to handle filenames with spaces

[MegaphoneJon]

Description

Converting an HTML file to Word fails when the HTML contains an <img> tag whose <src> is a) a URL, and b) contains a space.

I'm not sure if there's a reliable way to test this, since it requires loading a file from a URL; the bug isn't present when a space exists in a file path. However, I confirm that this passes all existing tests.

Fixes # (none)

Checklist:

• I have run composer run-script check --timeout=0 and no errors were reported
• The new code is covered by unit tests (check build/coverage for coverage report)
• I have updated the documentation to describe the changes

[MegaphoneJon]

Relevant quote from the PHP docs for file_get_contents:

If you're opening a URI with special characters, such as spaces, you need to encode the URI with urlencode().

[coveralls]

Coverage decreased (-0.0005%) to 94.629% when pulling 49b3ba7 on MegaphoneJon:spaces-in-filenames into 06c2f56 on PHPOffice:develop.