Indices are now identified by exact name

[MarkusBonsch]

Closes #2465.

Simply added the exact = TRUE to
the line where indices are identified:
xo = attr(attr(x, 'index'), idxName, exact = TRUE)
This implies the following changes:

• It prevents an index test from accidentally being used for column test_col.
• It prevents changed row order after joins if the index that was used for the join contained more columns than were joined on, e.g. Index on x and y but join only on x. With the bugfix, the x__y index is not used anymore for joins on x only.

[codecov-io]

Codecov Report

Merging #2476 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #2476   +/-   ##
=======================================
  Coverage   91.57%   91.57%           
=======================================
  Files          63       63           
  Lines       12039    12039           
=======================================
  Hits        11025    11025           
  Misses       1014     1014

Impacted Files	Coverage Δ
R/data.table.R	97.16% <100%> (ø)	⬆️

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