Resolve lint errors

Author: FelixHerrmann

Sources/MultipartFormData/Boundary.swift

@@ -104,6 +104,6 @@ extension Boundary: CustomDebugStringConvertible {
 
 extension Boundary {
     internal var _value: String {
-        return String(decoding: _asciiData, as: UTF8.self) // UTF-8 representation is exactly equivalent to ASCII
+        return String(bytes: _asciiData, encoding: .ascii) ?? ""
     }
 }

Sources/MultipartFormData/Internal/String+helpers.swift

@@ -7,6 +7,7 @@
 
 extension String {
     internal init(_ staticString: StaticString) {
+        // swiftlint:disable:next optional_data_string_conversion
         self = staticString.withUTF8Buffer { String(decoding: $0, as: UTF8.self) }
     }
 }

Sources/MultipartFormData/MultipartFormData.swift

@@ -162,6 +162,6 @@ extension MultipartFormData {
 
 extension MultipartFormData: CustomDebugStringConvertible {
     public var debugDescription: String {
-        return String(decoding: contentType._data + ._crlf + ._crlf + httpBody, as: UTF8.self)
+        return String(bytes: contentType._data + ._crlf + ._crlf + httpBody, encoding: .utf8) ?? ""
     }
 }

Sources/MultipartFormData/Subpart.swift

@@ -80,7 +80,7 @@ extension Subpart {
 
 extension Subpart: CustomDebugStringConvertible {
     public var debugDescription: String {
-        return String(decoding: _data, as: UTF8.self)
+        return String(bytes: _data, encoding: .utf8) ?? ""
     }
 }