Document and test String.replace_prefix|suffix with empty match
Document and test String.replace_prefix|suffix with empty match

This commit adds documentation and tests for String.replace_prefix/3 and
String.replace_suffix/3 for when the "match" argument is an empty
string (in which case they just prepend/append the replacement to the
given string).

kelvinst/elixir,antipax/elixir,joshprice/elixir,lexmag/elixir,michalmuskala/elixir,beedub/elixir,beedub/elixir,kimshrier/elixir,elixir-lang/elixir,pedrosnk/elixir,gfvcastro/elixir,kimshrier/elixir,pedrosnk/elixir,lexmag/elixir,kelvinst/elixir,ggcampinho/elixir,antipax/elixir,ggcampinho/elixir,gfvcastro/elixir