Fix support for PAY_PER_REQUEST BillingMode when Model includes Global Secondary Indexes

[batmanppc]

Summary:

This PR fixes the option for ON_DEMAND throughput for creating tables using PAY_PER_REQUEST Billing Mode with Global Secondary Indexes.

Code sample:

Schema

var User = new dynamoose.Schema({
  id: Number,
  name: { type: String, index: { global: true } }
}, {
  throughput: "ON_DEMAND"
});

GitHub linked issue:

Closes #503

Type (select 1):

• Bug fix
• Feature implementation
• Documentation improvement
• Testing improvement

• Test added to report bug (GitHub issue #--- @---)
• Something not listed here

Is this a breaking change? (select 1):

• 🚨 YES 🚨
• No
• I'm not sure

Is this ready to be merged into Dynamoose? (select 1):

• Yes
• No

Are all the tests currently passing on this PR? (select 1):

• Yes
• No

Other:

• I have searched through the GitHub pull requests to ensure this PR has not already been submitted
• I have updated the Dynamoose documentation (if required) given the changes I made
• I have added/updated the Dynamoose test cases (if required) given the changes I made
• I have run npm test from the root of the project directory to ensure all tests continue to pass
• I agree that all changes made in this pull request may be distributed and are made available in accordance with the Dynamoose license
• All of my commits and commit messages are detailed, explain what changes were made, and are easy to follow and understand
• I have confirmed that all my code changes are indented properly using 2 spaces
• I have filled out all fields above

[coveralls]

Pull Request Test Coverage Report for Build 801

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 85.381%

---

Totals
Change from base Build 791:	0.04%
Covered Lines:	2147
Relevant Lines:	2438

---

💛 - Coveralls

[fishcharlie]

LGTM