writeVarint with a string argument can result in an invalid message

[kjvalencik]

Example:

message Envelope {
    int32 type = 1;
    string name = 2;
}

/* ... */
var Envelope = compile(proto).Envelope;
var pbf = new Pbf();

Envelope.write({
    type: 'foo',
    name: 'test'
});

Envelope.read(new Pbf(pbf.finish()))

Result: Error: Unimplemented type: 4

It seems to be due to val being being coerced into NaN. I have not tested if other number types are affected.

A simple solution would be to force it to always be a number:

// Current
val = +val;

// Replacement
val = +val || 0;

But, maybe it would be better to if (isNaN(val)) throw new Error()?

[kjvalencik]

Oddly enough, while testing adding a throw, I found that isNaN is really slow.

This reduces performance in the encode benchmark by about 25%.

if (isNaN(val)) throw new Error('Expected number');

On the other hand, this has almost no impact:

if (!(val < 0) && !(val >= 0)) throw new Error('Expected number');

[mourner]

I'd prefer val || 0 — we don't want to care about validating input too much, because the main priority of this library is performance and simplicity.