Dropdown.Button as a single non-wrappable element

[alexkirsz]

When a Dropdown.Button is rendered inside of an element that isn't wide enough to contain both the button itself as well as the dropdown button, the text wraps around and the elements are displayed on different lines.

This PR adds a single white-space: nowrap; rule so that the elements are always displayed on the same line.

In the example below, the Dropdown.Button is displayed inside a table cell with width: ".1%", to push it to the far right.

Before:

After:

[mention-bot]

@Morhaus, thanks for your PR! By analyzing the history of the files in this pull request, we identified @afc163, @yiminghe and @PinkyJie to be potential reviewers.

[coveralls]

Coverage remained the same at 71.523% when pulling 072155b on Morhaus:patch-1 into 72e1364 on ant-design:master.

[afc163]

Thx!