RxScala: Fix errors in Completeness.scala and also improve it

[zsxwing]

• Fixed some errors in CompletenessTest.scala and improve it.
• Fixed the error of defer for Remove Variance on Defer #1561

/cc @headinthebox, @samuelgruetter

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[samuelgruetter]

LGTM

[samuelgruetter]

except one detail: in the CompletenessTest, replace

Note:
*    This table contains both static methods and instance methods.
*    If a signature is too long, move your mouse over it to get the full signature.

by

Note:

*    This table contains both static methods and instance methods.
*    If a signature is too long, move your mouse over it to get the full signature.

because apparently, that blank line is needed for the parser to recognize the bullet points.

[zsxwing]

@samuelgruetter done

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