fix double ctor in JSONWriter

[stleary]

This replaces #414 which had a coding standards error (tab instead of spaces). Also just noticed we don't have any coding standards published. Will update the Wiki.

[stleary]

What problem does this code solve?
The Double ctor is deprecated, so changed to with the recommended replacement method. This appears to only be an OpenJDK issue, but no harm in maintaining compatibility. For more information, see:
https://bugs.openjdk.java.net/browse/JDK-8176335.
This code was originally submitted in #414 but that change used a tab char, which has been corrected.

Risks
Low

Changes to the API?
No

Will this require a new release?
No

Should the documentation be updated?
No

Does it break the unit tests?
No. Confirmed that the existing tests already provide coverage for the modified line.

Was any code refactored in this commit?
No

Review status
APPROVED (#414 approval transferred to this pull request) Starting 3 day comment window.