message: chore(core): pull supported request methods out of constructor
  Also fixes incorrect return type on the BitGoRequest `end()` method,
  which has always returned `void`.
  Ticket:
diff: @@ -42,6 +42,8 @@ import { GlobalCoinFactory } from './v2/coinFactory';
  const debug = debugLib('bitgo:index');
  +const supportedRequestMethods = ['get', 'post', 'put', 'del', 'patch'] as const;
  +
  if (!(process as any).browser) {
  require('superagent-proxy')(superagent);
  }
  @@ -387,7 +389,7 @@ export interface VerifyPushTokenOptions {
  export interface BitGoRequest extends superagent.Request {
  result: (optionalField?: string) => Bluebird<any>;
  - end: (callback?: NodeCallback<superagent.Response>) => Bluebird<superagent.Response>;
  + end: (callback?: NodeCallback<superagent.Response>) => void;
  }
  export interface BitGo {
  @@ -544,9 +546,6 @@ export class BitGo {
  // functions that use it.
  this._validate = params.validate === undefined ? true : params.validate;
  - // Create superagent methods specific to this BitGo instance.
  - const methods = ['get', 'post', 'put', 'del', 'patch'];
  -
  if (!params.proxy && process.env.BITGO_USE_PROXY) {
  params.proxy = process.env.BITGO_USE_PROXY;
  }
  @@ -557,8 +556,8 @@ export class BitGo {
  this._proxy = params.proxy;
  - for (const index in methods) {
  - const method = methods[index];
  + // Create superagent methods specific to this BitGo instance.
  + for (const method of supportedRequestMethods) {
  this[method] = this.createPatch(method);
  }
  @@ -580,7 +579,7 @@ export class BitGo {
  * headers to any outbound request.
  * @param method
  */
  - private createPatch(method: string): (url: string, callback?: NodeCallback<superagent.Response>) => superagent.Request {
  + private createPatch(method: typeof supportedRequestMethods[number]): (url: string, callback?: NodeCallback<superagent.Response>) => superagent.Request {
  const self = this;
  return function (...args) {
  let req: superagent.SuperAgentRequest = superagent[method].apply(null, args);
  @@ -590,7 +589,7 @@ export class BitGo {
  // Patch superagent to return promises
  const prototypicalEnd = req.end;
  - req.end = function() {
  + req.end = function (): void {
  const thisReq: superagent.SuperAgentRequest = this;
  // intercept a request before it's submitted to the server for v2 authentication (based on token)
  thisReq.set('BitGo-SDK-Version', self.version());
  