Fix Filter / where clause without column names is removed in optimization pass

[Dandandan]

Which issue does this PR close?

Closes#220

Rationale for this change

DataFusion gives wrong results for filters that don't reference any column

What changes are included in this PR?

Workarounds the issue by not pushing down predicates that do not reference any column (1=2, FALSE, TRUE).
As we should be able to remove these anyway with constant folding and removing/replacing WHERE FALSE/ WHERE TRUE, I think this is a good option.

Are there any user-facing changes?

No, results should be correct now.

[codecov-commenter]

Codecov Report

Merging #225 (94c5b3a) into master (e86ad26) will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@ Coverage Diff @@## master #225 +/- ##
==========================================
+ Coverage 76.30% 76.43% +0.12% 
==========================================
Files 134 135 +1 Lines 23170 23264 +94 ==========================================
+ Hits 17681 17782 +101 + Misses 5489 5482 -7 

Impacted Files	Coverage Δ
datafusion/src/optimizer/filter_push_down.rs	97.73% <100.00%> (+0.07%)	⬆️
datafusion/tests/sql.rs	99.88% <100.00%> (+1.42%)	⬆️
datafusion/src/catalog/information_schema.rs	86.72% <0.00%> (-2.06%)	⬇️
datafusion/src/physical_plan/hash_join.rs	85.94% <0.00%> (-0.49%)	⬇️
datafusion/src/execution/context.rs	92.63% <0.00%> (-0.31%)	⬇️
ballista/rust/executor/src/main.rs	0.00% <0.00%> (ø)
datafusion/src/datasource/datasource.rs	100.00% <0.00%> (ø)
datafusion/src/optimizer/eliminate_limit.rs	88.88% <0.00%> (ø)

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[jorgecarleitao]

Looks great, ready to sail! :)

[alamb]

Thanks @Dandandan