Add validation to status code

.adr-dir

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+docs/adr

docs/adr/0002-http-model-conventions.md

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+# 2. HTTP model conventions
+
+Date: 2019-10-24
+
+## Context
+
+The classes which form the HTTP model (in the `sttp.model` package) vary in the conventions they use for error-handling
+and the parsing interface they offer.
+
+## Decision
+
+We want to unify the behavior of the HTTP model classes so that they work in a predictable and similar way.
+The conventions are:
+
+* `.toString` returns a representation of the model class in a format as in an HTTP request/response. For example,
+  for an uri this will be `http://...`, for a header `[name]: [value]`, etc.
+* constructors of the model classes are private; instances should be created through methods on the companion objects.
+* `[SthCompanionObject].parse(serialized: String): Either[String, Sth]`: returns an error message or an instance of
+  the model class
+* `[SthCompanionObject].apply(values)`: creates an instance of the model class; validates the input values and in case
+  of an error, *throws an exception*. An error could be e.g. that the input values contain characters outside of
+  the allowed range
+* `[SthCompanionObject].validated(...): Either[String, Sth]`: same as above, but doesn't throw exceptions. Instead,
+  returns an error message or the model class instance
+* `[SthCompanionObject].notValidated(...): Sth`: creates the model type, without validation, and without throwing
+  exceptions 

model/shared/src/main/scala/sttp/model/StatusCode.scala

@@ -1,6 +1,6 @@
 package sttp.model
 
-case class StatusCode(code: Int) extends AnyVal {
+class StatusCode private (val code: Int) extends AnyVal {
   def isInformational: Boolean = code / 100 == 1
   def isSuccess: Boolean = code / 100 == 2
   def isRedirect: Boolean = code / 100 == 3
@@ -10,7 +10,14 @@ case class StatusCode(code: Int) extends AnyVal {
   override def toString: String = code.toString
 }
 
-object StatusCode extends StatusCodes
+object StatusCode extends StatusCodes {
+  def apply(code: Int): StatusCode = validated(code).fold(e => throw new IllegalArgumentException(e), identity)
+  def validated(code: Int): Either[String, StatusCode] = {
+    if (code < 100 || code > 599) Left(s"Status code outside of the allowed range 100-599: $code")
+    else Right(new StatusCode(code))
+  }
+  def notValidated(code: Int) = new StatusCode(code)
+}
 
 // https://www.iana.org/assignments/http-status-codes/http-status-codes.xhtml
 trait StatusCodes {

model/shared/src/test/scala/sttp/model/StatusCodeTests.scala

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+package sttp.model
+
+import org.scalatest.{FlatSpec, Matchers}
+
+class StatusCodeTests extends FlatSpec with Matchers {
+  it should "return a string description of the status code" in {
+    StatusCode.Accepted.toString shouldBe "202"
+  }
+
+  it should "validate status codes" in {
+    StatusCode.validated(8) shouldBe 'left
+    StatusCode.validated(200) shouldBe Right(StatusCode.Ok)
+  }
+
+  it should "throw exceptions on invalid status codes" in {
+    an[IllegalArgumentException] shouldBe thrownBy(StatusCode(8))
+  }
+}