Improve img alt attribute for screen readers

Currently, the img_alt method in ActionView keeps underscores
in the alt attribute. Because underscores are pronounced in
Apple's VoiceOver Utility, this has serious implications for
accessibility. This patch makes underscored or hyphenated file
names (both common in projects) read more naturally in screen
readers by replacing them with spaces. See method documentation
for details.

Added documentation to image_alt method
Merge pull request #9221 from thenickcox/image_alt_attribute

Improve img alt attribute for screen readers