Support the server to run on Android API <=25

**Ktor Version and Engine Used (client or server and name)**
io.ktor:ktor-server-netty:1.2.5

**Describe the bug**
Ktor not work on Android API =< 25

**To Reproduce**
When i try to create server i get error:
```
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.myapplication, PID: 5399
java.lang.NoClassDefFoundError: Failed resolution of: Ljava/time/ZoneId;
at io.ktor.features.DefaultHeaders.(DefaultHeaders.kt:70)
```
ZoneId was added in android just in API 26

Start by reproducing the server startup failure on Android API 25 or lower and inspect the reference to DefaultHeaders.kt:70 shown in the stack trace. Done means the Ktor server can start on Android API <=25 without the java.time.ZoneId NoClassDefFoundError.

Written by the indexing model from the issue text.