Returns correct session name when used custom name and call `Session::getName()` before session open

[vjik]

Q	A
Is bugfix?	✔️
New feature?	❌
Breaks BC?	❌
Fixed issues	#45

[codecov[bot]]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage :thumbsup:

Coverage data is based on head (bea3b1d) compared to base (b2552a8). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #46 +/- ## =========================================== Coverage 100.00% 100.00% - Complexity 75 76 +1 =========================================== Files 3 3 Lines 183 187 +4 =========================================== + Hits 183 187 +4 ``` | [Impacted Files](https://codecov.io/gh/yiisoft/session/pull/46?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft) | Coverage Δ | | |---|---|---| | [src/Session.php](https://codecov.io/gh/yiisoft/session/pull/46/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft#diff-c3JjL1Nlc3Npb24ucGhw) | `100.00% <100.00%> (ø)` | | | [src/SessionMiddleware.php](https://codecov.io/gh/yiisoft/session/pull/46/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft#diff-c3JjL1Nlc3Npb25NaWRkbGV3YXJlLnBocA==) | `100.00% <0.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft)

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