fix: Allow for multiple values for a parameter

[xiaoboa]

This PR is trying to fix issue: sendgrid/sendgrid-ruby#145

Issue Summary

When using the client to get statistics for categories, it's not possible to retrieve stats for all categories

Steps to Reproduce

fun_stats = sg.client.categories.stats.get(query_params: {
    'start_date' => "2017-07-31",
    'categories' => 'category1&category2'
})

The above code makes a GET to this URL. Sendgrid returns stats for category1 only

https://api.sendgrid.com/v3/categories/stats?categories=category1&category2&start_date=2017-07-30

When the correct behaviour is a GET with both category1 and category2 as values to categories. Sendgrid returns the expected response.

https://api.sendgrid.com/v3/categories/stats?categories=category1&categories=category2&start_date=2017-07-30

This seems to be a bug, as the Sendgrid docs clearly state sending multiple categories is supported, and there doesn't seem to be a way to do that with this client.

Technical details:

sendgrid-ruby Version: 5.0.0
Ruby Version: 2.3.4

[SendGridDX]

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[thinkingserious]

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