Use passed in device_type variable instead of non-existent instance v…

[connor-odoherty]

What does this do and why?

Uses the passed in device_type string instead of the unset instance variable that is currently used in NamedUser.associate and NamedUser.disassociate.

Before this, attempting to associate a mobile channel ID for iOS devices resulted in a 404

https://github.com/urbanairship/ruby-library/issues/113

Additional notes for reviewers

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Testing

• [ ] I wrote tests covering these changes

• I've tested for Ruby versions:

• [ ] 2.2.5

• [ ] 2.3.1

• [ x ] 2.4.4

Urban Airship Contribution Agreement

https://docs.urbanairship.com/contribution-agreement/

• [ x ] I've filled out and signed UA's contribution agreement form.

Screenshots

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