Fix linear gaussian test

CanonicalGaussianTest#constructor_linear_gaussian is currently failing.

Failed tests:   constructor_linear_gaussian(dk.bayes.math.gaussian.CanonicalGaussianTest): expected:<-1.828012123484645[4]> but was:<-1.828012123484645[1]>

The test is concise so here is an obvious fix.
Is there a reason not for doing so? Performance? Better numeric stability?
Merge pull request #1 from BertrandDechoux/test-fix

Fix linear gaussian test