Unwrap non-null types when computing table name (#24)

This allows for fields like this:

    type Thing {
      metadata: [Pair!]!
    }

    type Pair {
      key: String!
      value: String!
    }

Author: jgwhite

addon/fields/records.js

@@ -1,11 +1,12 @@
 import { getNodeField } from '../relay/node';
 import { getTableNameForField } from '../db';
+import { unwrapNonNull } from '../utils';
 
 export const composeGetRecordsByFieldName = (getNodeField, getTableNameForField) =>
   (fieldName, field, db, options = {}) => {
     let { fieldsMap = {} } = options;
     let nodeField = getNodeField(fieldName, field);
-    let typeName = (nodeField || field).type.name;
+    let typeName = unwrapNonNull((nodeField || field).type).name;
     let tableName = getTableNameForField(fieldName, field.parent, typeName, fieldsMap);
     let table = db[tableName] || [{}];