LCOV reporter with toHTML = false does only report last file

[lgrammel]

When reporting to HTML, a diff gets appended for each file (lcov_reporter line 26). However, when I tried set toHTML to false, the window._$blanket_LCOV variable is overridden for each file (lcov_reporter line 28).

It should probably be appended as well, e.g.
window._$blanket_LCOV = (window._$blanket_LCOV || '') + str;