[ujson] Fixing seg faults in unit tests

- The following unit tests were causing segfaults:
```
test_doubleLongDecimalIssue
test_invalidDoublePrecision
test_doublePrecisionTest
test_encodeDecimal
test_doubleLongIssue
test_encodeDecodeLongDecimal
```
- I changed a constant that shouldn't be `const` because it can be overwritten by
  passing in a parameter (double_precision).  I believe this was first introduced in #106.
- Fixes the issue described in #129