[Proposal] Add contentDisposition to Response::download

So we can inline files (like images etc), instead of `Response::download($file)->setContentDisposition('inline')`
Check whether a view lacks a bound variable

This is useful when I want to make a form add an option based on the level of the user. This test allows me to check that that information is missing when the current user does not have the required level.
More minor cs fixes
make test fail if a view is not returned
Merge pull request #3919 from GrahamCampbell/4.1-fixes

More Minor CS Fixes
Merge pull request #3917 from ravloony/4.1

[Proposal] Add an assertion that checks whether a view lacks a given variable. (Was against master branch)
Working on comments. Updating change log.
Merge pull request #3910 from barryvdh/patch-1

[Proposal] Add contentDisposition to Response::download
Shorten variable name.
Add --heads option to sub-tree split.