Squares instead of spaces

[ekino-dgp]

In the recent builds, the spaces are replaced by the missing glyph indicator (squares in Firefox, squares with a '?' in Chrome) for some documents.

This is an improvement since previous versions didn't show any text at all, but we're not quite there yet.
Current pdf.js built-in version in Firefox v35 shows no text content for our test doc.

You can check a testcase at:
http://mozilla.github.io/pdf.js/web/viewer.html?file=http://ringabell.org/pdf_ko.pdf
current version is 1.0.1130, and the described behavior is present since at least 1.0.990

This issue seems close to #5564 but it's not the exact same, as the test document provided there shows the missing glyph indicators in the current built-in viewer, whereas our testcase just shows no text at all.