Update library/swift/HeadersBuilder.swift

Co-authored-by: JP Simard <[email protected]>

Signed-off-by: Rafał Augustyniak <[email protected]>

library/swift/HeadersBuilder.swift

@@ -4,7 +4,7 @@ private let kRestrictedPrefixes = [":", "x-envoy-mobile"]
 
 private func isRestrictedHeader(name: String) -> Bool {
   let isHostHeader = name.caseInsensitiveCompare("host") == .orderedSame
-  let hasRestrictedPrefix = kRestrictedPrefixes
+  lazy var hasRestrictedPrefix = kRestrictedPrefixes
     .contains { name.range(of: $0, options: [.caseInsensitive, .anchored]) != nil }
   return isHostHeader || hasRestrictedPrefix
 }