storage: strip leading slash in File() constructor

[cjihrig]

This commit prevents a double leading slash from being included in the uploaded file name when bucket.file() is called with a path starting with a slash. Only a single slash is removed, because it is assumed that if multiple leading slashes are provided, then the user may have done so on purpose.

Fixes: #1972

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[cjihrig]

I signed it!

[googlebot]

CLAs look good, thanks!

[coveralls]

Coverage remained the same at 100.0% when pulling ab7b967 on cjihrig:slash into a0341b2 on GoogleCloudPlatform:master.

[stephenplusplus]

Thank you @cjihrig! I made a small commit with a style change. If that doesn't break, I will merge straight away.

[coveralls]

Coverage remained the same at 100.0% when pulling a77e07e on cjihrig:slash into a0341b2 on GoogleCloudPlatform:master.