fixes fit(LassoPath,@formula,df) interface and adds tests for it

[AsafManela]

Previously the StatsModels.DataFramesRegressionModel was incompatible with Lasso.jl's using a keyword argument for the intercept, so fit(LassoPath, formula(Y~X), df) did not work properly.

fit(LassoModel, @formula(Y~X), df; select=MinAIC()) did not work at all.

This PR fixes that issue and adds tests for both.

[coveralls]

Pull Request Test Coverage Report for Build 162

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 88.618%

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Totals
Change from base Build 161:	0.2%
Covered Lines:	545
Relevant Lines:	615

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 166

• 11 of 12 (91.67%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 88.548%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/segselect.jl	10	11	90.91%

Totals
Change from base Build 161:	0.1%
Covered Lines:	549
Relevant Lines:	620

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💛 - Coveralls