dont monkeypatch String with #snakecase

[pcai]

Bug report

Current behavior:
savon should avoid monkeypatching a new method String#snakecase on load. this behavior is undocumented, specific to savon's needs as a library, and shouldn't unexpectedly change behavior of code the host application relies on.

Steps to reproduce current behavior:

gem install savon
irb

then:

"asdf".snakecase
(irb):1:in `<main>': undefined method `snakecase' for "asdf":String (NoMethodError)

require 'savon'
=> true

"asdf".snakecase
=> "asdf"

Expected behavior:
savon does not monkeypatch String on load.

System information:

• ruby version: all
• savon version: all

[pcai]

related: savonrb/wasabi#59