Add nullvalue

[xxlv]

Description:

I have noticed that the go-graphql project does not handle null values for input correctly. According to the GraphQL specification GraphQL Spec, there are two scenarios for handling null values in inputs:

1. When the input is required, null is not an acceptable value.
2. When the input parameter is optional, the null literal is acceptable.

For example, the following is not allowed:

Def:

NonNullField(arg: String!): String

Use:

{
    NonNullField(arg: null) // not allowed
}

After completing my changes, I found that @krasish has done similar work in pull request #683 , which has not yet been merged.

Please review this implementation, thank you very much.

[coveralls]

coverage: 92.056% (+0.008%) from 92.048%
when pulling d0c8027 on xxlv:add-nullvalue
into f2b39ca on graphql-go:master.

[coveralls]

coverage: 92.056% (+0.008%) from 92.048%
when pulling c5ca6e6 on xxlv:add-nullvalue
into f2b39ca on graphql-go:master.

[coveralls]

coverage: 92.072% (+0.02%) from 92.048%
when pulling 72d7caa on xxlv:add-nullvalue
into f2b39ca on graphql-go:master.