Short-circuit replace for Strings when pattern is not found

[sostock]

This simply returns the original string if the pattern is not found (instead of allocating a new IOBuffer). This is considerably faster if the pattern is not found.

Before:

julia> @btime replace("abc", "e" => "f")
  173.027 ns (6 allocations: 288 bytes)
"abc"

After:

julia> @btime replace("abc", "e" => "f")
  37.549 ns (0 allocations: 0 bytes)
"abc"