Convert the path to snakecase for each call

Convert the path to each operation to snakecase in `builder#use_namespace`.  

It appears that the format of these strings were always lower camel case, whereas `@operation_name` is in snakecase.  This would result in no matches being found when there were multiple namespaces.  Converting the string to snakecase (hopefully) resolves this issue.

Related to: [Issue #384](//github.com/savonrb/savon/issues/384)

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