[@types/caseless] Add Httpified interface. (#22935)

To allow importing libs to extend the interface.
Objects passed into `caseless.httpify` are extended with these methods.

types/caseless/caseless-tests.ts

@@ -1,4 +1,4 @@
-import caseless, { Caseless, httpify } from "caseless";
+import caseless, { Caseless, httpify, Httpified } from "caseless";
 
 new Caseless(); // $ExpectError
 
@@ -38,3 +38,20 @@ httpify({}, 1); // $ExpectError
 httpify({}, "2"); // $ExpectError
 
 httpify({}, {}); // $ExpectType Caseless
+
+const request: Httpified = {}; // $ExpectError
+
+request.setHeader({}); // $ExpectType void
+request.setHeader('foo', 'bar'); // $ExpectType string | false
+request.setHeader('foo', 'bar', false); // $ExpectType string | false
+request.setHeader('foo', new Date()); // $ExpectType string | false
+request.setHeader(10, 'bar'); // $ExpectError
+
+request.getHeader('foo'); // $ExpectType any
+request.getHeader(10); // $ExpectError
+
+request.hasHeader('foo'); // $ExpectType string | false
+request.hasHeader(10); // $ExpectError
+
+request.removeHeader('foo'); // $ExpectType boolean
+request.headers = {};

types/caseless/index.d.ts

@@ -1,6 +1,7 @@
 // Type definitions for caseless 0.12
 // Project: https://github.com/mikeal/caseless
 // Definitions by: downace <https://github.com/downace>
+//                 Matt R. Wilson <https://github.com/mastermatt>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 // TypeScript Version: 2.2
 
@@ -17,6 +18,15 @@ export interface Caseless {
     del(name: KeyType): boolean;
 }
 
+export interface Httpified {
+  headers: RawDict;
+  setHeader(name: KeyType, value: ValueType, clobber?: boolean): KeyType | false;
+  setHeader(dict: RawDict): void;
+  hasHeader(name: KeyType): KeyType | false;
+  getHeader(name: KeyType): ValueType | undefined;
+  removeHeader(name: KeyType): boolean;
+}
+
 export function httpify(resp: object, headers: RawDict): Caseless;
 
 declare function caseless(dict?: RawDict): Caseless;