Improve tests and change robustize behavior

[shntnu]

• add new test for robustize
• for robustize normalization, force to a data frame because of this issue https://github.com/tidyverse/dbplyr/issues/357#issuecomment-54885081
• other edits to increase test coverage

[codecov[bot]]

Codecov Report

Merging #141 into master will increase coverage by 9.52%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #141      +/-   ##
==========================================
+ Coverage   89.94%   99.47%   +9.52%     
==========================================
  Files          17       17              
  Lines         378      381       +3     
==========================================
+ Hits          340      379      +39     
+ Misses         38        2      -36

Impacted Files	Coverage Δ
R/variance_threshold.R	100% <ø> (+6.25%)	:arrow_up:
R/variable_importance.R	66.66% <100%> (+66.66%)	:arrow_up:
R/normalize.R	100% <100%> (+7.14%)	:arrow_up:
R/variable_select.R	100% <100%> (+26.66%)	:arrow_up:
R/transform.R	100% <100%> (+71.42%)	:arrow_up:
R/aggregate.R	100% <100%> (+7.14%)	:arrow_up:
... and 2 more

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