Check response type and body before calling childs

I have run into this, and added a checks before calling or checking childs.

> TypeError: Cannot call method 'indexOf' of undefined
at Object.livereload (/Users/dejanr/Projects/.../node_modules/koa-livereload/index.js:13:28)

> TypeError: Cannot read property 'pipe' of undefined
 at Object.livereload (/Users/dejanr/Projects/.../node_modules/koa-livereload/index.js:27:25)