Bug 877141 - Strip down non-break spaces from contenteditable

When several consecutives spaces are typed in a contenteditable element,
all but the last one are converted to non-break spaces. We don't want
this, here, so we simply replace them with normal spaces.
Merge pull request #10333 from lissyx/bug877141

Bug 877141 - Strip down non-break spaces from contenteditable

r=julien