Space gets duplicated

Space gets duplicated

Like https://github.com/eemeli/yaml/issues/56, not an issue that we encountered in the wild but one that my property-based test uncovered.

```
> YAML.stringify([{"key1":[],"key2":"!\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"#\"\\ '"}])
'- key1:\n    []\n  key2: "!\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"#\\"\\\\\n    \\ \'"\n'

> YAML.parse('- key1:\n    []\n  key2: "!\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"\\"#\\"\\\\\n    \\ \'"\n')
[ { key1: [],
    key2: '!""""""""""""""""""""""""""""""""""#"\\  \'' } ]
                                                   ^~~~ extra space inserted here
```

------

I updated the test script to `return true` in case of parsing errors, and passed some arguments to `fc.string()` to control for the length. Had to run it a couple of times to come up with this error.


```ts
import fc = require('fast-check');
import YAML = require('yaml');
import deepEqual = require('deep-equal');
import { Arbitrary } from 'fast-check';

// Unfortunately it's hard to generate fully recursive type definitions
// so we'll unroll by hand a couple of times

function makeRecord<T, U>(a: Arbitrary<T>) {
  return fc.record({
    key1: a,
    key2: a
  });
}

const arbitraryJson0 = fc.oneof<string|number|boolean|null>(
  fc.string(0, 100),
  fc.integer(),
  fc.float(),
  fc.boolean(),
  fc.constant(null),
);


const arbitraryJson1 = fc.oneof<any>(
  arbitraryJson0,
  fc.array(arbitraryJson0),
  makeRecord(arbitraryJson0)
);

const arbitraryJson2 = fc.oneof<any>(
  arbitraryJson1,
  fc.array(arbitraryJson1),
  makeRecord(arbitraryJson1),
);

const arbitraryJson3 = fc.oneof<any>(
  arbitraryJson2,
  fc.array(arbitraryJson2),
  makeRecord(arbitraryJson2),
);


// Property #1: validate that stringifying and parsing are each other's inverse
// Do a manual sampling and test so that we can continue even if we find failing cases.
const N = 100000;

fc.assert(fc.property(arbitraryJson3, fc.context(), (original, context) => {
  const rendered = YAML.stringify(original);
  try {
    const parsed = YAML.parse(rendered);
    context.log('Parsed as ' + JSON.stringify(parsed));
    return deepEqual(original, parsed);
  } catch(e) {
    // Throwing is also bad but not what we're looking for right now
    return true;
  }
}), {
  numRuns: N
});
```