Add support for charset attribute in search

[Paxa]

This allows to use non-ascii characters in search, e.g. Café

With this fix can be used as:

gmail.inbox.emails(gm: "Café".force_encoding('ascii-8bit'), charset: "UTF-8")

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Solution inspired by dcparker/ruby-gmail#81

Would it be nice to apply UTF-8 as charset automatically?

[coveralls]

Coverage remained the same at 90.056% when pulling f53f95b on Paxa:master into c5e9389 on gmailgem:master.

[coveralls]

Coverage remained the same at 90.056% when pulling f53f95b on Paxa:master into c5e9389 on gmailgem:master.

[coveralls]

Coverage remained the same at 90.056% when pulling f53f95b on Paxa:master into c5e9389 on gmailgem:master.

[coveralls]

Coverage remained the same at 90.056% when pulling f53f95b on Paxa:master into c5e9389 on gmailgem:master.

[coveralls]

Coverage remained the same at 90.056% when pulling f53f95b on Paxa:master into c5e9389 on gmailgem:master.

[johnnyshields]

As of version 0.7.0 (Aug 19, 2018) this gem is officially deprecated and will no longer be maintained. Please instead use Google's official Gmail API Ruby Client, which uses the Gmail API rather than IMAP and has significantly better performance and reliability.