Normalize trailing slash in store URLs

[omandryk]

Summary

• integrate the trailing-slash normalization contributed by @ekonopliov in Fixed URL formatting issue #32
• preserve the contributor's original commit and authorship
• add focused regression coverage for URLs with and without a trailing slash

Why

A store URL ending in / currently produces a double slash when the API path is appended. The contributor fix removes exactly one trailing slash during OAuthConfig construction.

Compatibility

• no public API or binary signature changes
• no changes to OAuth signature generation
• URLs without a trailing slash are unchanged

Verification

• mvn clean verify -Dgpg.skip=true
• 9 tests, 0 failures, 0 errors, 7 existing live-store tests skipped
• JAR, sources JAR, and Javadocs JAR built successfully

Credits @ekonopliov for the original fix in #32.