Fixing expr_creates_var failure in R Programming
merging with master; fixing trivial conflict

Merge branch 'master' into expr_creates_var_failure

Conflicts:
	R_Programming/Matrices_and_Data_Frames/customTests.R
Use testthat::is_true() rather than is_true() in customTests.R. This seems necessary.
correcting inadvertant overwrite of custom test calculates_same_value
cleared e$delta when either custom test fails
Custom omnitest which clears e$delta upon failure.
Merge pull request #20 from swirldev/expr_creates_var_failure

Fix for expr creates var failure in R Programming