Fix an error message StartsWith parser throws

Sources/Parsing/ParserPrinters/StartsWith.swift

@@ -39,6 +39,7 @@
 public struct StartsWith<Input: Collection>: Parser where Input.SubSequence == Input {
   public let count: Int
   public let possiblePrefix: AnyCollection<Input.Element>
+  public let originalPossiblePrefix: Any
   public let startsWith: (Input) -> Bool
 
   /// Initializes a parser that successfully returns `Void` when the initial elements of its input
@@ -57,13 +58,14 @@ public struct StartsWith<Input: Collection>: Parser where Input.SubSequence == I
   where PossiblePrefix.Element == Input.Element {
     self.count = possiblePrefix.count
     self.possiblePrefix = AnyCollection(possiblePrefix)
+    self.originalPossiblePrefix = possiblePrefix
     self.startsWith = { input in input.starts(with: possiblePrefix, by: areEquivalent) }
   }
 
   @inlinable
   public func parse(_ input: inout Input) throws {
     guard self.startsWith(input) else {
-      throw ParsingError.expectedInput(formatValue(self.possiblePrefix), at: input)
+      throw ParsingError.expectedInput(formatValue(self.originalPossiblePrefix), at: input)
     }
     input.removeFirst(self.count)
   }

Tests/ParsingTests/StartsWithTests.swift

@@ -7,4 +7,19 @@ final class StartsWithTests: XCTestCase {
     XCTAssertNoThrow(try StartsWith("Hello".utf8).parse(&str))
     XCTAssertEqual(", world!", Substring(str))
   }
+
+  func testParseFailure() {
+    var input = "Goodnight, Blob!"[...].utf8
+    XCTAssertThrowsError(try StartsWith("Hello, ".utf8).parse(&input)) { error in
+      XCTAssertEqual(
+        """
+        error: unexpected input
+         --> input:1:1
+        1 | Goodnight, Blob!
+          | ^ expected "Hello, "
+        """,
+        "\(error)"
+      )
+    }
+  }
 }