fix: types: pathParams can be initially undef

one might wish to write a client like this:

```js
class Foo extends Gofer {
  constructor(cfg) {
    super(cfg, 'foo');
  }

  bar({ x, y }) {
    return this.get('/{x}/{y}', {
      endpointName: 'bar',
      pathParams: { x, y },
    });
  }
}

const foo = new Foo({ foo: { pathParams: { x: 'whee' } } });
await foo.bar({ y: 'yay' });
```

with the current types, it will complain that you're passing an
undefined value as a pathParam, but really that will be dealt with in
the config merging later on, so we should accept it

lib/typedefs.d.ts

@@ -42,10 +42,13 @@ declare class Gofer {
 }
 
 declare namespace Gofer {
-  export function fetch(path: string, opts?: Gofer.FetchOpts): FetchResponse;
+  export function fetch(
+    path: string,
+    opts?: Gofer.FetchOpts & { pathParams?: { [param: string]: string } }
+  ): FetchResponse;
 
   export type Opts = {
-    pathParams?: { [param: string]: string };
+    pathParams?: { [param: string]: string | undefined };
     timeout?: number;
     connectTimeout?: number;
     searchDomain?: string;