Custom message not always respected

[artsoroka]

Inside "closeTo" function "this" already references an Assertion with "msg" parameter previously being set, so no need to call new Assertion

Fixes #923

[meeber]

Hi @artsoroka. Thanks for the PR. Although this solution does technically resolve the problem with .closeTo, it's not really compatible with #922, and would need to be refactored again as soon as #922 is merged. Therefore, the current plan is to wait until #922 is merged and then fix all the custom message issues in one go.