Have Environment.set(_:value:) actually set the environment (#557)

* Have Environment.set actually set the environment

* Add test

Author: adam-fowler

Sources/Hummingbird/Environment.swift

@@ -92,6 +92,11 @@ public struct Environment: Sendable, Decodable, ExpressibleByDictionaryLiteral {
     ///   - value: Environment variable name value
     public mutating func set(_ s: String, value: String?) {
         self.values[s.lowercased()] = value
+        if let value {
+            setenv(s, value, 1)
+        } else {
+            unsetenv(s)
+        }
     }
 
     /// Merge two environment variable sets together and return result

Tests/HummingbirdTests/EnvironmentTests.swift

@@ -41,8 +41,16 @@ final class EnvironmentTests: XCTestCase {
         XCTAssertEqual(env.get("TEST_VAR"), "testSet")
     }
 
+    func testSetForAllEnvironments() {
+        var env = Environment()
+        env.set("TEST_VAR", value: "testSet")
+        let env2 = Environment()
+        XCTAssertEqual(env2.get("TEST_VAR"), "testSet")
+    }
+
     func testLogLevel() {
-        setenv("LOG_LEVEL", "trace", 1)
+        var env = Environment()
+        env.set("LOG_LEVEL", value: "trace")
         let router = Router()
         let app = Application(responder: router.buildResponder())
         XCTAssertEqual(app.logger.logLevel, .trace)