Add NaN support

[mkgrgis]

This implementation methodologically bases on

• ISO:SQL
• NaN values converted to 0 #36
• Many problems with formal text affinity values in foreign tables #66
• SQLite NaN ignore code

This PR implements NaN as readable value from SQLite if there is such case insensitive value with text affinity.

According ISO:SQL and implemented PostgreSQL code we can input NaN as special IEEE-754 value through API or as special text constant through SQL. SQLite doesn't support NaN with real affinity (in opposition to ±∞ values) and doesn't rewrites NULL value if NaN is detected. Hence in this PR NaN is readable and detectable but not writeable.

About changes:

• NaN implementation was untested outside of aggregates test, 70f2f36 adds tests for previous implementation,
• 026d6ef adds support for readable NaN value with text affinity in SQLite.

[mkgrgis]

Hello, @t-kataym ! Which calendar plans have pgspider team to review this PR?

[MinhLA1410]

Hello @mkgrgis ,
Thank you for your support.
I have reviewed your PR. Could you please check it?

[mkgrgis]

I have reviewed your PR. Could you please check it?

Yes, @MinhLA1410 ! All of your comments are checked. All problems should be fixed in 9413bed. Please verify.

[mkgrgis]

After merging of this PR #36 can be closed.

[mkgrgis]

@MinhLA1410 , any updates?

[MinhLA1410]

@mkgrgis Thank you. I have no futher comment.
I will leave the decision to @t-kataym

[t-kataym]

@mkgrgis Thank you for your contribution. I will merge it.