[aws|put_object] guard against non-us_ascii x-amz-meta-* values

As http://docs.aws.amazon.com/AmazonS3/latest/dev/UsingMetadata.html states

>  Each name, value pair must conform to US-ASCII when using REST.

This constraint is especially hard get, since sending non-us_ascii characters in
a `x-amz-meta-*` header value simply results in a signing error.
[aws|put_object] do not check user meta data encoding for ruby-1.8.7

- There is no `Encoding` module in `ruby-1.8.7`
- `fog v2` will drop support for `ruby-1.8.7`
- Simply revert this commit once support for `ruby-1.8.7`is dropped
Merge pull request #3344 from janraasch/feature/fail-on-non-us_ascii-user-meta-data

[aws|put_object] guard against non-us_ascii x-amz-meta-* values