Add safety for reserved words in buildNet

[VitaliyR]

Hi!

I was playing around with my pet project - and suddenly added word constructor as matcher for buildNet:

const matches = [
  { match: 'constructor' }
]
const net = nlp.buildNet(matches)

And when tried to run the code - it crashed because it tries to call push method on object.constructor which is a function.

I've fixed it via explicitly checking for types. What do you think?

P.S. Also not sure if thats the best place for test - feel free to point me into right direction. Thanks!

[spencermountain]

This is beautiful Vitalii!
thank you!